Structure of Poly(dialkylsiloxane) Melts:  Comparisons of Wide-Angle X-ray Scattering, Molecular Dynamics Simulations, and Integral Equation Theory

Abstract

Wide-angle X-ray scattering, molecular dynamics (MD) simulations, and integral equation theory are used to study the structure of poly(diethylsiloxane) (PDES), poly(ethylmethylsiloxane) (PEMS), and poly-(dimethylsiloxane) (PDMS) melts. The structure functions of PDES, PEMS, and PDMS are similar, but systematic trends in the intermolecular packing are observed. The local intramolecular structure is extracted from the experimental structure functions. The bond distances and bond angles obtained, including the large Si-O-Si angle, are in good agreement with the explicit atom (EA) and united atom (UA) potentials used in the simulations and theory and from other sources. Very good agreement is found between the MD simulations using the EA potentials and the experimental scattering results. Good agreement is also found between the polymer reference interaction site model (PRISM theory) and the UA MD simulations. The intermolecular structure is examined experimentally using an appropriately weighted radial distribution function and with theory and simulation using intermolecular site/site pair correlation functions. Experiment, simulation, and theory show systematic increases in the chain/chain packing distances in the siloxanes as the number of sites in the pendant side chains is increased.

Long Range Polymer Chain Dynamics of Highly Flexible Polysiloxane in Solution Probed by Pyrene Excimer Fluorescence

“…As anticipated from T g measurements of PDMS in the solid-state [ 10 , 18 ], we find that a polysiloxane backbone yields fast LRPCD in solution. Furthermore, this study confirms that the Model Free Analysis of TRF decays of polymers randomly labeled with pyrene yields reliable and quantitative information about the relative magnitudes of the dynamic processes experienced by different polymeric chains in solution.…”

“…While it is reasonable and expected that < k MF > blob for PDMS, a polymer well-known for its flexibility [10,18], is greater than for PS in THF, the 2.9 fold difference is much smaller than that reported for the dynamics of pyrene end-labeled monodisperse PDMS and PS polymers in toluene [19]. Thus, the fact that the LRPCD probed by PDMS constructs that are monodisperse and end-labeled are thrice the value for polydisperse and randomly labeled PDMS with pyrene may be a consequence of the different bond angles of the PDMS backbone: the Si–O–Si bond angle is 146° whereas that of O–Si–O is 108° [18]. This difference implies that a PDMS chain in its most energetically stable all trans -conformation would be coiled, whereas the backbone of a PS chain in the same all trans -conformation would be fully extended because its C–C–C bond angles are near 109°.…”

PVA-PDMS-Stearic acid composite nanofibrous mats with improved mechanical behavior for selective filtering applications

“…In experimental and simulation XRD studies on PDMS melt structures, a broad peak has been observed at q = 0.85 Å −1 (where q = 4π sin θ /λ) which for CuK α radiation corresponds to 2 θ ~ 12°. This peak has been attributed to intermolecular correlations in liquid state PDMS chains in bulk 40 , 41 , where chains are relatively ordered in liquid state. In our case, a nano-scale confinement due to hydrophilic PVA, the intermolecular distance could further be reduced and lead to a reflection at an increased value of 2 θ .…”

A systematic procedure to build a relaxed dense-phase atomistic representation of a complex amorphous polymer using a coarse-grained modeling approach

“…It is recognized that these initial dense-phase models do not yet represent a truly equilibrated structure of the polymer. Additional advanced sampling algorithms are needed and are being planned to attain this level of equilibration, with the equilibrated polymer structures to be validated by comparing RDFs from the polymer model with RDFs obtained by wide-angle x-ray scattering (WAXS) measurements [38, 39]. The present work sets the stage from which these further studies can be implemented to attain our final objective of producing accurate dense-phase all-atom models of complex amorphous polymers, which can then be used as a guide for polymer design to optimize performance for targeted applications.…”